1. Technical Field
The present disclosure relates to an over-current protection circuit and pulse width modulator having the same, and in particular, to an over-current protection circuit, and a pulse width modulator having the same, capable of preventing from overcurrent during turn-on of the high-side switch.
2. Description of Related Art
Generally speaking, a pulse width modulator is a DC-DC voltage converter, and modulates the inputted dc voltage to a voltage level by stepping, so as to provide a stable load voltage to peripheral elements.
Please refer to FIG. 1 which is a schematic view of a pulse width modulator in the prior art. As shown in FIG. 1, a pulse width modulator 10 comprises an output-stage element 11, a trigger 13 and a driving module. The driving module comprises a feedback element 12, a minimum-on-time element 14, a judging element 15, a control element 16, a high-side overcurrent detecting element 17 and a current sensing element 18. The output-stage element 11 is operated under a power source VCC, and the output-stage element 11 comprises a high-side switch US, a low-side switch LS, an inductor L and a capacitor C. One end of the high-side switch US is connected electrically to an input end for receiving the power source VCC. Another end of the high-side switch US is connected electrically to one end of the low-side switch LS for forming a connection node SW. Another end of the low-side switch LS is grounded. One end of the inductor L is connected electrically to the connection node SW, and another end of the inductor L is connected electrically to an output end for outputting a load voltage VOUT. One end of the capacitor C and one end of the feedback element 12 are connected electrically between the inductor L and the output end. Another end of the capacitor C and another end of the feedback element 12 are grounded. The feedback element 12 is a voltage divider circuit to detect the load voltage VOUT and generate a feedback signal FB to the judging element 15 correspondingly.
The trigger 13 is configured for receiving a pulse signal CLK to generate a set signal SET to the control element 16 periodically. The high-side current sensing element 18 is connected electrically to the input end and the connection node SW to sense the inductive current IL flowing through the high-side switch US which is drawn in solid line, and transmit the result of sensing the inductive current IL flowing through the high-side switch US (i.e., the solid line) to the high-side overcurrent detecting element 17 and the judging unit 15. The high-side overcurrent detecting element 17 judges whether the inductive current IL flowing through the high-side switch US (i.e., the solid line) has the overcurrent, and generates a high-side overcurrent signal HS_OCP to the judging element 15 when the high-side overcurrent detecting element 17 judges that the inductive current IL flowing through the high-side switch US (i.e., the solid line) has the overcurrent.
The judging element 15 generates a pulse width signal DUTY according to the feedback signal FB and the result of sensing the inductive current IL flowing through the high-side switch US (i.e., the solid line), to adjust the duty cycle of the high-side switch US and the low-side switch LS correspondingly. The judging element 15 generates a reset signal RESET according to the pulse width signal DUTY and the high-side overcurrent signal HS_OCP. Therefore, the control element 16 can generate a high-side turn-on signal UGON and a low-side turn-on signal LGON according to the set signal SET and the reset signal RESET, to periodically control the high-side switch US and the low-side switch LS, to turn on or off the high-side switch US, and turn on or off the low-side switch, so as to adjust the inductive current IL flowing through the high-side switch US which is drawn in the solid line, and the inductive current IL flowing through the low-side switch LS which is drawn in the dashed line.
However, a parasitic inductance exists at the connection node SW after the high-side switch US is turned on for a period, and it causes the ringing effect to be generated in the voltage of the connection node SW. When the current sensing element 18 senses the inductive current IL after turn-on of the high-side switch US for a period, the current sensing element 18 will sense the wrong inductive current IL. In general control systems, blanking is utilized to avoid the ringing effect, so as to prevent the high-side overcurrent detecting element 17 from receiving the wrong inductive current IL and misjudging. In practice, a minimum-on-time element 14 is added to the pulse width modulator 10 in the prior art. The minimum-on-time element 14 receives the pulse signal CLK, to periodically generate a minimum on time MINTON which corresponds to the period after the high-side switch US has been turned on, so that the turn-on of the high-side switch US can be maintained for the minimum on time MINTON after the control element 16 turns on the high-side switch US.
The minimum-on-time element 14 controls the current sensing element 18 according to the high-side turn-on signal UGON, during which the high-side switch US is turned on within the minimum on time MINTON to prevent the control element 16 from being affected by the high-side overcurrent signal HS_OCP and the pulse width signal DUTY, to turn off the high-side switch US, for example, the reset signal RESET is maintained at low level during the minimum on time MINTON. Therefore, the high-side overcurrent detecting element 17 is prevented from receiving the wrong inductive current IL and making the high-side switch US act mistakenly. After the minimum on time MINTON, the current sensing element 18 senses whether the inductive current IL is larger than an overcurrent level LEV_OCP as shown in FIG. 2, to provide the sensing result to the high-side overcurrent detecting element 17 for judging whether the inductive current IL has the overcurrent. The high-side overcurrent detecting element 17 generates the high-side overcurrent signal HS_OCP when the inductive current IL is detected to be larger than the overcurrent level LEV_OCP.
Please refer to both of FIG. 1 and FIG. 2, the FIG. 2 is a signal waveform of the pulse width modulator in prior art. Based on the structure of the pulse width modulator 10 in prior art, within time T1, the inductive current IL is larger than the overcurrent level LEV_OCP, and the high-side switch US is turned on within the minimum on time MINTON. The current sensing element 18 does not generate the high-side overcurrent signal HS_OCP, so the inductive current IL increases continuously. Next, within time T2, high side current sense function is enabled. The overcurrent of high-side is detected and the current sensing element 18 generates the high-side overcurrent signal HS_OCP to make the inductive current IL decrease. Within time T3-T4 and time T5-T6, the current sensing element 18 also repeats the operation within the time T1-T2, respectively.
However, the increasing speed of the inductive current IL of the pulse width modulator 10 is faster than decreasing speed and the high-side switch US is not turned off within the minimum on time MINTON. Therefore, the inductive current IL still increases continuously even if the inductive current IL is the overcurrent within the minimum on time MINTON such as time T1, T3 and T5, so that the average current of the inductive current IL increases gradually as time goes on, such as the average current of the inductive current IL within time T1-T6. Moreover, when the output voltage is too low that the output is judged to be shorted to ground, an under voltage protection component in the pulse width modulator 10, which is not shown in the figure, usually generates an under voltage protection signal UVP to make the inductive current IL decrease continuously, such as the under voltage protection signal UVP within time T1-T6. Therefore, if the inductive current IL in the pulse width modulator 10 becomes too high before the under voltage protection signal UVP is generated, for example, the inductive current IL increases to the position P, the pulse width modulator 10 may be burned out.